<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8" />
    <title>3.姓名案例_watch实现</title>
    <!-- 引入Vue -->
    <script type="text/javascript" src="../js/vue.js"></script>
</head>

<body>
    <!-- 准备好一个容器-->
    <div id="demo">
        姓：<input type="text" v-model="firstName"> <br>
        名：<input type="text" v-model="lastName"> <br>
        姓名：<span>{{fullName}}</span>
    </div>
</body>

<script type="text/javascript">
    //阻止 vue 在启动时生成生产提示。
    Vue.config.productionTip = false
    new Vue({
        el: '#demo',
        data: {
            firstName: 'zhang',
            lastName: 'san',
            fullName: ''
        },
        watch: {
            firstName: {
                immediate: true,
                handler(val) {
                    this.fullName = val + '_' + this.lastName
                }
            },
            lastName(val) {
                this.fullName = this.firstName + '_' + val
            }
        }
    })
</script>

</html>